Conventional electronic devices employing inorganic materials use physical properties of bulk materials, typically as crystalline silicon. When the scale of miniaturization reaches the utmost limit, it becomes impossible to obtain the bulk properties and difficult to obtain a desired function. On the other hand, organic materials can assign a function to one molecule and expand them into the device element size, and thereby they can exhibit the desired function.
Organic materials include various compounds having a base of a carbon skeleton. Among them, it was verified that organic molecules with conductivity exhibit diverse electric properties derived from their molecular structures, and proposals have been made for application on various organic electronic devices, such as thin film transistors, sensors, organic LEDs, capacitors, batteries, bio-functional devices, and lasers.
Thin film transistors (hereinafter also referred to as “TFTs”) currently are expected to be useful drive elements for devices, such as active matrix liquid crystal displays. TFTs normally are formed of inorganic semiconductor materials, such as amorphous silicon and low-temperature polysilicon. Forming a semiconductor layer of a TFT by organic molecules enables reducing manufacturing costs and enlarging panel areas.
However, the organic semiconductors already been reported to have problems, such as having lower carrier mobility than those of inorganic semiconductors and causing high driving voltage when applied to device elements. For this reason, research has been carried out to improve the carrier mobility of organic semiconductors and to lower the driving voltage of device elements that employ organic semiconductors.
In most organic electronic devices that employ conductive organic molecules, sufficient device characteristics expected from the properties of conductive organic molecules can not be obtained. One possible factor is considered to be insufficient connectivity between electrodes and conductive organic molecules. As one of the methods to improve their electric connectivity, a method for easy charge transfer from an electrode to an organic semiconductor by disposing an organic layer (an electron transport layer or a positive hole transport layer) between the electrode and the organic semiconductor layer and for lowering the driving voltage of an organic TFT has been disclosed (JP10 (1998)-125924 A). A method of coating an electrode by any of a metal film, a conductive polymer film or a monomolecular film chemically bonded to the electrode for the purpose of reducing connection resistance between a conductive organic thin film and the electrode is disclosed (JP2003-309307).
The method described in JP10 (1998)-125924 A, however, requires a step, such as patterning, to form the organic layer at a predetermined position because the organic layer (the electron transport layer or the positive hole transport layer) is disposed as an intermediate layer independent from the electrode and the semiconductor layer. In addition, conventional techniques did not achieve sufficient improvement in electric connectivity between an electrode and an organic molecule layer, and thus a wide range of variations was found in the obtained characteristics in some cases.